1. Technical Field
This invention relates generally to a valve packing arrangement such as may be used around a valve stem to keep fluids from leaking out of the valve along the stem. More particularly, the invention is concerned with a packing arrangement which provides for adjustment in the compression of a packing element between the valve body and the valve stem.
2. Background Information
In meeting the requirements of the Environmental Protection Agency, it is important that fugitive emissions from chemical plants be kept to a minimum. One source of fugitive emissions is the leakage of fluids along the stems of valves used in controlling the process fluids of the plant. Special packing materials such as polytetrafluoroethylene and perfluoroelastomer have been found to meet governmental sealing requirements in reducing fluid losses from valves. Polytetrafluoroethylene, however, suffers from plastic flow under certain conditions requiring that the compression of the packing be adjusted from time to time in order to tightly seal between the valve stem and valve body. On the other hand, perfluoroelastomer seals have more elastomeric properties which at normal operating temperatures do not exhibit plastic flow. However, with temperature increases volumetric swell of perfluoroelastomer packings can cause packing stress in a confined volume and upon cool down may result in fluid leakage.
One prior arrangement valve packing arrangement is shown in U.S. Pat. No. 5,056,757 wherein there is described a live load packing system which includes V-type polytetrafluoroethylene (PTFE) packing rings which are in a loaded condition. Specifically, on each side of the packing rings there is an anti-extrusion ring. One end of a packing follower engages one of the anti-extrusion rings and is urged to compress the packing rings by a series of Bellville disc springs mounted at the other end of the packing follower. The Bellville disc springs are maintained in position on the packing follower by means of an O-ring. A packing flange engageably contacts one end of the Bellville disc springs and packing nuts may be tightened so that the packing flange transmits compressive force to the Bellville disc springs to load the packing rings and seal between the valve stem and valve body.
Another arrangement for loading sealing rings around a reciprocating plunger is shown in U.S. Pat. No. 3,011,808. As shown in FIG. 2 of that patent, a modified loading member is used to bias the sealing rings in the proper manner to effect a more efficient seal for a longer period of time. In the disclosed arrangement, a member includes a rigid main portion with a spring loaded inner ring. The latter is biased by a series of coil springs against a point on the lip of the sealing members when a packing gland is tightened to produce design precompression of the assembly.